The chemical combination of two or more incompatible polymers into sequential copolymers, i.e., block and graft copolymers, often leads to a unique combination of physical properties not originally present in either of the two component polymers or in their physical blends.
Poly(vinyl chloride) (PVC) has been one of the most widely used vinyl polymers in the world for more than 70 years. Graft copolymers derived from PVC are new materials whose physical properties may be improved considerably over those of PVC itself. Since the 1960s, anionic, cationic, and free-radical graft copolymerizations of PVC have been studied.
Anionic graft copolymers of PVC can generally be obtained from nucleophilic substitution reactions of chlorine atoms. As a result, a polymeric anion is grafted onto the PVC backbone. Appropriate displacement agents are characterized by a strongly nucleophilic character, while their basicity should be low in order to avoid base-promoted dehydrochlorination. In order to avoid undesirable termination of activity, air and polar species generally must be excluded, a major limitation of anionic grafting of PVC.
Cationic grafting involves the formation of a carbocation on the polymer backbone via abstraction of a chloride anion by a Lewis acid. Initiation of graft copolymerization then takes place from the polymeric cations, which form primarily at labile halogen sites such as allylic and tertiary chlorides. It may be preferable to increase the number of these labile sites by the prior dehydrochlorination of PVC or by the use of a copolymer of vinyl chloride with a monomer such as 2-chloropropene. However, these processes increase production costs and also introduce structural defects which, if not removed completely by graft formation, will decrease the thermal stability.
Cationic PVC grafting catalysts include the trialkylaluminums and dialkylaluminum monohalides. In general, for synthesis of PVC grafts, techniques using alkylaluminums (such as Et3AI) are superior to earlier methods employing conventional Friedel-Crafts halides (such as Et2AlCl), because the former processes are more readily controllable, so that gelation and degradation can thus be easily minimized or avoided. Hence, the products are cleaner and consequently easier to analyze. The electrophilic reactivity of polymeric cations precludes the use of monomers containing electron-withdrawing groups for cationic polymerization.
Free-radical grafting onto PVC is applicable to a larger number of monomers than the anionic and cationic methods. However, the resulting graft copolymers are always contaminated by a significant amount of free homopolymer; whereas both the anionic and cationic methods afford well-defined graft copolymers. This problem occurs because many types of initiator radicals will add competitively to the monomer that is to be grafted, thereby initiating homopolymerization during the free-radical grafting process.
Another disadvantage of the conventional radical graft copolymerization is intrinsic to the structure of PVC. If a chloromethylene hydrogen is abstracted, the branch point resulting from grafting with any monomer will incorporate tertiary chloride and thus be thermally labile.
Moreover, if a methylene hydrogen is abstracted, the resultant carbon-centered radical, instead of adding to the graftable monomer, may simply undergo a thermal loss of a chlorine atom to yield an unstable allylic chloride.
In summary, conventional free-radical grafting on PVC will introduce structural defects that will decrease the thermal stability of the resulting polymer.
Recently, U.S. Pat. No. 6,437,044 describes living radical graft polymerization from the structural defects of PVC by using metal catalysts. Well-defined PVC graft copolymers can be initiated directly from the structural defects available in the PVC backbone by radical abstraction of chlorine. Suitable catalysts include iron, cobalt, nickel, copper, ruthenium, rhodium, palladium and salts thereof, including iron chloride, iron bromide, nickel chloride, ruthenium chloride, rhodium chloride, and palladium acetate. Preferably, copper catalysts are utilized, including Cu2O, CuCl, CuBr, Cu2S, and Cu2Se. Suitable monomers include methyl methacrylate, butyl methacrylate, tert-butyl methacrylate, butyl acrylate, methacrylonitrile, acrylonitrile, styrene, 4-chlorostyrene, 4-methylstyrene, and isobornyl methacrylate. However, the resulting copolymers are contaminated by the residual metal, and their compositions depend on the number of reactive structural defects in the starting PVC, or require starting PVC materials that are specially made to have a high number of allylic and/or tertiary chloride sites. Since the typical PVC formulation generally has from about 1 to about 5 and more often from about 2 to about 3 labile chlorine sites per 1,000 repeat units of PVC, there are limited branching sites.
There remains a need for a novel method of producing graft copolymers from PVC that lacks the deficiencies of the prior art.